Aircraft propeller control system



Deco 9, 1952 P. J. QLHNN 2,620,883

AIRCRAFT PROPELLER CONTROL SYSTEM Filed Oct. 1, 1945 2 SHEETS-SI-IEET 1 .lfaizg'fild 4 Pressure 3nnentor R Gttorneg II I l I I l N m U Q In R AIRCRAFT PROPELLER CONTROL SYSTEM Imnentor faul .f Um'mz I (Iitorneg Filed 001;. 1, 1945 Patented Dec. 9, 1952 AIRCRAFT PROPELLER CONTROL SYSTEM Paul J. Quinn, South Orange, N. J., assignor to Curtiss-Wright Corporation, a corporation of Delaware Application October 1, 1945, Serial No. 619,714

8 Claims.

This invention relates to speed control systems for prime movers and more particularly to highly sensitive governor systems wherein correction is provided for unusual conditions rendering the normal operation of the governor unsatisfactory.

In such speed control systems, the time lag relationship between the speed correcting control device and engine response results in overactuation of the control before the engine reaches the desired speed, unless anticipatory or acceleration sensitive means are provided to terminate actuation of the control device a proper interval of time prior to the prime mover reaching the desired speed. By such an arrangement, hunting is avoided and the control device may require no further actuation to hold the prime mover at the steady speed ultimately attained in response to such actuation. In applications of Chillson and Schoenbaum, Serial No. 593,525, filed May 12, 1945, and Quinn, Serial No. 593,527, filed May 12, 1945, there are disclosed such acceleration and anticipatory means for rendering a governor highly sensitive to the control setting required for maintaining a desired speed. Governors also are available whose sensitivities are manually adjustable so that damping is attained at a value to match the response lag of the governed system in its usual state of operation. The three types above mentioned may all be considered as damped governor systems.

Under extreme conditions, the time lag relationship between the operation'of the speed control means and the response of the prime mover varies to such an extent that the precision operation of such systems no longer obtains. While the rate of change of the control means is usually essentially constant, various factors surrounding the operation of a prime mover and its load have been found to undergo substantial variation when unusual or extreme conditions exist which produce a substantial change in the time lag relationship. In an internal combustion engine-propeller combination the indicated airspeed and intake manifold pressure are found to be indices of the time lag relationship.

Stated in another manner, a governor may be made sensitive to small offspeed indications, but the stability of the governed system depends upon the damping characteristics of the governor being in proper relation to the lag of the governed system to avoid under-correction or over-correction. In most ground-operated systems, the system lag is substantially uniform so that uniform governor damping is satisfactory. But in aircraft power plants, for example, the lag of a (Cl. fill-135.72)

propeller system may vary according to the speed level at which the power plant operates, the airspeed of the aircraft, the power delivered by the power plant, the altitude and other factors. Thus to secure the proper stability in the governed system and thus proper balance between governor damping, and load lag under all conditions, it becomes desirable to alter the governor damping in consonance with changes which occur in the known factors affecting load lag.

It is accordingly an object of this invention to vary the constants of a governor system for a prime mover and connected load in response to one or more indices indicative of substantial variation in conditions affecting or accompanying changes in the constants of the prime mover and connected load.

A further object of the invention is to provide in such a controlling system means simultaneously sensitive to a plurality of factors determinative of the time interval between cessation of operation of the control system and the attaining of a desired equilibrium speed of the prime mover.

Another object of the invention is to provide a system for attaining the foregoing objects by a system capable of adaptation to electronic circuits.

Still another object of the invention is toprovide in an engine-propeller combination a control system sensitive to the time lag characteristics of the control and engine, and in which compensation is provided for substantial changes in the time lag characteristics of the combination due to airspeed or extreme engine load and speed conditions or both.

Yet another object of the invention is to pro.- vide a control system of the class set forth in which simplicity and ruggedness is present and in which at the same time the prime mover speed may be controlled and maintained with a high degree of precision and dispatch.

The above and other objects and novel features of the invention will appear more fully hereinafter from the following detailed description when taken in conjunction with the accompanying drawings. It is expressly understood, however, that the drawings are employed for purposes of illustration only and are not designed as definitions of the limitations of the invention, reference being had for this purpose to the appended claims.

In the drawings, wherein like reference characters indicate like parts:

Figure 1 illustrates a speed control system for an engine propeller combination in which a plu- 3 rality of factors coact to establish or maintain a desired speed; and

Figure 2 illustrates an alternative speed control system of a simplified form employing an acceleration ressponsive circuit.

Referring to Figs. 1 and 2 of the drawings, there will be seen a prime mover and connected load generally designated by the reference character 8, the load as illustrated comprising a variable pitch propeller ID with a pitch-changing mechanism I2 adapted to vary the pitch of the blades of the propeller to vary the speed of or the load upon the prime mover. In such combinations, the prime mover may constitute an. internal combustion engine, the throttle of which can be fixed at any desired setting during desired steady operation of the combination, or varied at will when substantial changes in power or torque output are desired. The pitch-changing mechanism illustrated is of the electric type although any type may be employed.

For the purpose of maintaining a desired speed, or causing the pitch of the propeller blades to change in order to change to a different desired speed, a tuned circuit HI is employed, one of the constants of which may be varied in order to establish a tuned frequency bearing a relationship to the speed of the prime mover to be maintained. The prime mover is provided with a twophase alternator I6 directly driven thereby for producing an alternating current whose frequency varies in direct proportion to the speed of the prime mover and whose frequency throughout the speed range of the prime mover is within the range of adjustment of the frequency of the tuned circuit I4.

By means of electronic circuits, a'signal voltage E is produced when the resonant frequency of the tuned. circuit I4 differs from the frequency of the alternator I6, which signal voltage is employed in conjunction with compensating means to cause actuation of the pitch-changing motor in such a manner as to change the load upon the prime mover to bring the prime mover to the desired speed as indicated by the adjustment of the tuned circuit I4. It will be appreciated that in an engine-propeller combination of the type set forth and in prime mover load combinations generally, there is a lag in effecting a change of speed between the actuation of the speed controlling mechanism and the subsequent attainment of the desired speed of the prime mover in response to actuation of such control and each of the circuits herein disclosed is adapted to compensate for such lag, in order to obviate hunting of the system. Not only is hunting avoided but additional factors having to do with the time lag of the system are compensated for to the end that speed corrections are effected with a minimum of delay.

The pitch-changing gear may be similar to that disclosed in U. S. Patent 1,951,320, the mechanism comprising a reversible motor I! adapted to be operated in one direction or the other in response to the closing of the contacts of either of the relays IB or of' Figure 1 or the actuation of the differential relay 22 of Figure 2 so as to close either one or the other contact thereof, the circuit in either figure being such as to deliver current from a battery 24 through slip rings 26 to one or the other of forward and reverse fields 28 and 30, and at the same time release a magnetic brake 32. The pitch-changing motor upon energization rotates the propeller blades about their axes in their respective sockets, the drive there- 4 for being effected through a high ratio reduction gear 34.

To establish a signal voltage E, the direction and magnitude of which is a true indication of the difference between the adjusted frequency of the tuned circuit I4 and the frequency of the alternator IS, the direction being in accordance with whether the alternator frequency is greater or less than that of the frequency adjustment of the tuned circuit, a discriminator circuit is employed. Such a circuit operating from two phases 36 and 38 at 90 degrees phase relation to one another of the alternator I6 is so connected that one phase 36 is applied, through a transformer 40 having oppositely wound secondaries 42 and 44, to oppositely energize the plates 46 and 48 of the twin triode vacuum tube 50 through the resistances 52 and 54, the center tap of which is connected to the common cathode connection of tube 50. The other alternator phase 38, having a 90- degree relationship to phase 36 is provided with a transformer 39 having a secondary I I connected through the parallel resonant or tuned circuit I4 to both grids and 62 in the twin triode 50 through grid current limiting resistors 64 and 66. A high resistance shunt 65 between grids and cathodes is also provided. When zero phase shift occurs through the tuned circuit II, that is, the alternator frequency is resonant with the tuned circuit, the flow of current through the resistors 52 and 54, and filtering capacities 56 and 58, as thus far described, is equal and opposite, resulting in a zero voltage across both resistors at E.

When the alternator frequency is higher or lower than that ofv the tuned circuit M, the tuned circuit will cause a phase shift in the voltage from 90 degrees out of phase with both plate secondary voltages 42 and 44, to an unbalanced condition in which the grid voltage is more in phase with one plate than the other, depending upon whether the frequency is higher or lower, thus causing a higher current flow through one of the resistors 52 and 54 and a lower current through the other. The balance in the twin triode 50 is thus upset and a difference voltage E created Whose direction and magnitude depends upon the phase shift and degree thereof. Thus far the modifications of Figures 1 and 2 are similar.

Referring particularly to Figure 1, the signal voltage E is passed through a filter network composed of a resistance or inductance 68 and capacity "I0, adapted to attenuate stray frequencies and thence to a load resistance I2. The load resistance "I2 is connected at its opposite ends through grid current limiting resistors I4 and 16 to the grids I8 and 8B of the twin triode 82, the connection to the resistor I6, however, including a pair of resistors 84 and 86, across which opposed potential drops are impressed. When the op-- posed potential drops are equal. the resistances in effect offset one another and the voltages thereacross are neutralized so that the filtered voltage Es is impressed across the grids I8 and of the twin triode 82.

Grid load resistors 88 and 90 are connected across the lines leading to the grids and grid limiting resistors I4 and I6. The center tap between resistors 88' and 90 is connected through an automatic bias resistor 92 to both cathodes 94 and 96 of the dual triode 82. The plates 98 and I00 are connected to the opposed solenoids or windings I02 and IM of a differential relay I06 and the common return of the solenoids is connected to a source of direct current furnished by the transformer secondary I08 andone-half of a twin rectifier III]. The source of current is provided with a filter I99 and is connected to the mid-connection between the grid return resistors 88 and 90 and through the cathode bias resistor 92 to the cathodes of the twin triode 82. Assuming the voltages across the resistors 84 and 86 equal and opposite when a voltage E exists the voltages impressed upon one of the grids i6 and 89 will be positive and the other negative and a greater current will be caused to pass through the plate circuit having the more positive grid, resulting in one ofthe solenoids I92 and I94 of the differential relay overpowering the other to close the contact H2 or II4 with the switch blade H6. Thereupon either of the relays I8 or from the battery 24 is energized operating the pitch-changing motor IT to effect a corresponding change inpropeller pitch.

Since as long as there exists a voltage Es, and assuming a high degree of sensitivity in the circuit thus far described, the pitch-changing motor will be caused to operate until such time as the frequency of the generator I6 and that of the tuned circuit I4 correspond and the engine is returned to on speed. Because of the time lag characteristics of the engine-propeller combination, the pitch angle, which will have been effected during the time interval occupied by the engine in arriving at the desired speed, will not be the pitch angle for maintaining the engine on tablished. When this occurs the frequency of the generator I6 changes beyond the frequency of the tuned circuit I4 creating a voltage Es in opposition to that formerly existing with the result that the pitch-changing motor is then caused to operate in a reverse direction. With the high degree of sensitivity possible in a system such as has been described, the pitch-changing mechanism may be caused to reverse in a definite cycle with the engine speed lagging behind and wandering or hunting above and below the desired speed, an extremely undesirable condition.

In order to forestall hunting, the system is provided with means to stop the change of pitch before the engine speed arrives at the speed desired, and in instances the arrangement is such that the pitch-change motor may momentarily be reversed before the desired engine speed is reached if conditions are such that the desired speed may be exceeded. Where the time lag relationship is increasingly long, rather than delay the achievement of the desired speed by stopping the pitch change a sufficient time in advance to allow gradual equilibrium to take place, the pitch may be purposely changed beyond that point required for equilibrium momentarily, and then reversed to bring the pitch to the equilibrium angle, at approximately the instant the engine reaches equilibrium speed for that angle.

To accomplish the foregoing in Figure 1, a synthetic feed back voltage is developed during energization of either of the pitch-changing relays I8 and 29 and applied to the voltage ES, by destroying the equalization of the opposed voltages in resistors 84 and 66- so that a differential voltage Er is set up in opposition to the voltage E5. The combined voltages Es and Er may be such as to neutralize one another. The voltage Er may exceed Es, and establish a reverse voltage across the grids I8 and 89. A twin triode II8, the plates I29 and I22 of which are connected to the opposite ends of resistors 84 and 86 is supplied with current from a secondary I24 of transformer 39, the other half of the twin rectifier III], and filter I I I, connections being made to the mid-connection between resistors 84 and 86 and the inter-connected cathodes I26 and I28. The grids I39 and I32 of the twin triode II8 are each connected through time delay circuits I34, each of which is composed of a capacity and resistors, the one comprising capacity I36 and resistors I38 and I40, and the other comprising capacity I42 and resistors I44 and I46.

To perform a novel feature of the invention, an engine manifold pressure-sensitive device I48 such as an expansible chamber bellows, actuates a double pole switch I 50 to short circuit resistors I49 and I46 of the time delay circuits. When the absolute pressure exceeds a certain value, usually present during normal flight conditions, the bellows actuates the contacts to decrease the time constant of the time delay circuits, and thereby affect the feed back voltage in a manner to be described hereinafter.

During steady on speed operation, the battery 24, energizing resistors I52 and IE4 through the coil I53 of relay I8, and resistors I56 and I58 through the coil I51 of relay 20 maintains a negative grid bias on both grids I39 and I32, so that the effective resistance of the tube I I6 is high and the currents flowing through the resistors 84 and 86 are equal and low, :the bias for both tubes being determined by th potential of the battery 24, and the relative values of the resistors I52 and I54 and I56 and I56. When the relay I96 is actuated to connect the switch blade I I 6 with either relay contact H2 or I14, either the resistors I52 and I54, or the resistors I56 or I58 are shunted and the negative grid bias of the corresponding grid is thereupon reduced to zero at a rate governed by the size of the corresponding capacity I35 or I42, and the resistors I38, I40 or I44 and I46. When the grid bias is reduced from a negative value to zero the plate current increases, the effective resistance of that half of the triode de creases, increasing the voltage drop across the corresponding resistor 84 o 86 to establish a differential voltage to be combined with the voltage Es as heretofore set forth.

By shunting resistors I49 and I46 when the engine manifold pressure is high, and removing the shunt when an abnormally low pressure exists, the time constant for discharging the capacity I36 or I42 is altered with a consequent effect on the rates of establishment of the synthetic differential voltage across resistors 84 and 86. The more quickly the synthetic voltage is built up, the sooner it neutralizes the signal voltage. When the combined voltage-s equal zero, the relay is opened and further pitch change is discontinued in the direction indicated as required by :the signal voltage.

Another factor bearing on the time lag constant is airspeed. When airspeed is relatively high (as at high altitude), the engine-propeller time lag constant is decreased, and the anticipatory time may accordingly be decreased. To ef fect this result, an airspeed sensitive device I69 connected to a pi tot tube is employed to actuate a relay I62 adapted symmetrically to decrease the portion of resistors 84 and 86, and the voltages thereacross, in circuit with the load resistance I2 and its signal potential, to decrease the opposing effect of the differential feedback voltage upon the potential E, and thereby shorten the anticipatory time. I

Analternative method for compensation is disclosed in the modification of Fig. 2; Therein, the signal voltage E acts upon an acceleration sensitive network comprising the series. of resistors I64, I66, and I68 and the capacity I16 and the load resistors I12 and: I14. Ihe serially connected load resistances I12. and I14 produce a voltage El across resistor I1 2 which will be. proportional to the total voltage drop E and, hence, proportional to the oir' speed of the engine. The resistances I64, I66 and I68 are connected through capacity I16 and the resistor I12 across the signal voltage E. The signal voltage E is impressed upon the grids I16 and I18 of the twin triode I86 through the grid resistors I82 and I84, the capacity I10 being interposed on the side of the grid I'16.

It will thus be apparent that the voltage. impressed across the gridsthrough their limiting. resistors will be that of the sum of the voltages E1 and the voltage drop at across resistors I64, I66 and I68. The voltage E1 as noted, is proportional to the engine off speed voltage E, and is zero when the engine is on speed. The voltage. e1 however, appears only when. the speed of the engine is changing, the magnitude of such voltage being approximately proportional to the rate of acceleration and the polarity being in one sense when the. acceleration is positive and in the opposite sense when the acceleration is negative.

The voltage el will therefore be added to or subtracted from the voltage E depending upon whether the engine speed is increasing or decreasing preferably being added when the engine speed is being increased above the reference speed and subtracted when the engine speed is being reduced from above the reference speed. The resistance I12 and the resistances I64, I66 and I68 acting conjointly thus provide a signal voltage for the grids of the twin triode I80, one 1 component of the signal voltage being proportional to and of the same sign as the off speed of the engine, and the second component being proportional to and of the same sign as the acceleration of the engine.

Serially connected grid resistors I86 and I88 for the grids of the twin triode I80 are connected to the grid limiting resistors I82 and I84, the center connection being connected to the cathodes I90 of the twin. triode.

I94, respectively, are connected to the coils I96 and I98 of the differential relay 22 and thence together are connected. to a plate voltage supply 2" consisting of a battery as illustrated, the negative terminal of which is connected to the cathode I90. Both sides of the twin triode are similarly biased so that when the signal voltage is zero (the engine being on speed and not changing speed) the flow of. current through the plate circuits will cause the coils I96 and I98 to cppose one another in such a manner that the relay 22 will be held in the neutral position out of contact with. the circuits to the pitch-changing motor open. As soon, however, as the grids of the twin triode are biased to a different degree the flow of current through the relay coils I96 and I88 will no longer be uniform and the relay 22 will be caused to energize the pitch-changing motor so as to operate in one direction or the other.

Owing to inertia, the instantaneous speed of an engine-propeller combination of the character described will lag its equilibrium speed during changes in speed in a manner known in the art. Hence, while the engine propeller combination is undergoing a change in speed, the voltage E1 The plates I92 and 8. across the resistor I12 being proportional only to engine off. speed, will also lag the equilibrium speed. To provide stable operation of the governing; system, the lag betweenthe engine speed and the equilibrium. speed. is. compensated for by the voltageei which is produced across the resistors I64, I66 and I68 wherever there is a. change in the: value of E dueto a change in speed. The resistances I64, I66 and. I68 and. the capacity H6 are. so selected. that the voltage e1 produced by changes in. E is approximately equal to the rate of. change of E and thus is a function of the acceleration of the engine. The ratio of E/El is so chosen that the sumxof EL and c1, when the engine is approaching governing speed, isa reasonably accurate index. of. the equilibrium speed. of: the engine for: most normalv conditions experienced in flight, the sum of E1 and e1 being connected to the relay twin triode so that the relay for controlling the pitch. change motor-maybe controlled in accordance with the equilibrium speed. of the engine propeller combination instead of the actual speed. In other words, in effecting any correction of the speed of the engine-propeller combination, the acceleration-sensitive network is operated to combine with the off speed voltage, a. voltage proportional to the rate of acceleration, whereby as the equilibrium speed oi the engine. propeller combination. reaches the on speed level, the voltage er across the resistances I64, I66 and .168 is equal and. opposite; to the voltage Er across the resistance I12. Hence the relay 22. will move to its neutral position die-energizing the. pitch change motor when the voltages E1 and. or are equal and opposite. Thereafter, as the enginepropeller combination progresses to an equilibrium speed the voltages across the resistance i1 2 and the resistances I64, I66 and I68 decay exponentially along. uniform. curves and are of opposite polarity. The relay will therefore remain in its neutral position during such period and for so long a period thereafter as the engine-propeller combination remains. on speed.

While the above circuit as described is satiatfactory for normal flight conditions, with increases in airspeed the engine-propeller time. constant decreases substantially such that the value of the combined resistors I64, I66 and I66 no longer create, a voltage er of. a. value equal to E1 at the proper instant when actuation of the pitchchanging motor should cease. In an internal combustion engine, it has also been iound, that during operation when the manifold pressure is abnormally low, the time constant of the enginepropeller combination is in a similar fashion increased. In each 01' these. extraordinary situations, it is desirable to alter the constants of the acceleration network composed of the resistors I64, I66 and I68 and the capacity I18- and there is thus provided a manifold pressure sensitive device 282' adapted under normal conditions to shortcircuit a, portion of the resistances I64, I66 and I68, the portion being represented by the resistance I66 so that upon a low manifold pressure the effect of the resistance. I66 is included in the circult to increase the time constant thereof. Similarly, a portion of the resistances I64, I66 and I66 and that part represented by the resistance I68 is adapted to be short-circuited upon an increase inairspeed above normal flight conditions so that the time constant of the circuit may be decreased during such conditions.

From the foregoing, it will be apparent that the systems described are operative to control an engine-propeller combination in accordance with its equilibrium speed and to be sensitive to the changes in the time constant of the combination during such abnormal conditions, the presence of which are indicated by reduced manifold pressure or high airspeed. Over-correction, with attendant hunting and constant use of the pitch-changing motor, is thus avoided while the time required to effect a desired correction is reduced to a minimum. It will be noted that the pitch of the propeller will be corrected when ever the engine speed deviates a predetermined extent from its on speed level regardless of the rate of drift and that the pitch will be corrected Whenever the engine accelerates or decelerates at a rate greater than a predetermined rate even though it may be on speed when such correction is initiated. In instances in which the rate of acceleration of the engine-propeller combination in the direction of the on speed level is such that the equilibrium speed will exceed such level, the acceleration voltage or the synthetic voltage being greater than that of the signal voltage or a voltage proportional thereto at such time, the operation of the pitch controlling relays will be such as to reverse the pitch change motor even though such reversal is counter to the action indicated alone by the off speed voltage. The systems therefore have the advantage of high flexibility and enable stable control under normal operating conditions as well as extreme conditions.

In the variation of Fig. 2 both the airspeed and the manifold pressure sensitive devices act on portions of a single resistance control unit, resulting in a simplification. Such simplification may be employed in connection with the arrangement shown in Fig. 1 by providing a system wherein both the airspeed and manifold pres sure sensitive devices act upon portions of the resistors 84 and 86. Additionally, if desired, both the airspeed and manifold pressure switches may be arranged to operate upon and vary the resistance of the time delay circuits I34 by varying the value of the resistances I38 and ME! and I44 and [45. It will also appear that the airspeed pressure sensitive device and the manifold pressure sensitive device may be interposed, that is, the manifold pressure device may be made to act upon the resistors 84 and 36 and the airspeed pressure device made to act upon the resistors of the time delay circuits I3 5. Under certain conditions of flight, it may be desirable to temporarily eliminate the automatic control of either manifold pressure or airspeed over the constants of the electrical circuits. For this purpose, a control switch such as l5! (Fig. 1) may be provided to open the shunting circuits about resistors I49 and M6. Similarly, a manually operable switch I 6| may be provided in the relay circuit actuated by the airspeed pressure sensitive device to eliminate, at will, the automatic operation thereof.

In each of the circuits illustrated the constants are varied by a single fixed increment sufiicient to approximate the required change, as to certain extreme conditions. While it will appear that such an arrangement is highly simple and desirable, nevertheless, the bellows acting in response to airspeed or manifold pressure can be made to actuate continuously variable resistor elements, variable in a degree proportional to the bellows movement and such as to provide a constant which varies in correspondence with the constant required for substantially exact equivalence over the range of conditions effecting the time lag relationship between the prime mover and its controls. It will also appear that while the constants have been illustrated as varied by airspeed, or manifold pressure, other measurable quantities present in a prime mover and connected load which reflect changes in the time lag relationship may be employed to vary the constants. Among such measurable quantities are engine thrust, engine torque, throttle position, rate of fuel flow, or the application of water injection as in internal combustion engine operation. Engine temperature, when reflecting load, can be employed, or the temperature of an element of the engine such as the temperature of a gas turbine blade and any one or combination of the foregoing and other variables, as will appear to those skilled in the art, may be employed.

Though several modifications have been illustrated and described, it will be understood that the invention is not limited to the precise disclosure, but may be embodied in various arrangements and equivalent circuits. As many changes may be made without departing from the spirit of the invention, as will be apparent to those skilled in the art, reference will be had to the appended claims for a definition of the limits of the invention.

I claim:

1. A system for controlling the speed of a power plant driving a propeller, the blades of which are adjustable as to pitch, a mechanism to adjust blade pitch to change the speed of said power plant from one speed to a different predetermined speed, means responsive to the actual speed of said power plant and in accordance with the rate of acceleration thereof for rendering said mechanism non-operative at a predetermined time interval before said power plant attains said predetermined speed level, and means responsive to airspeed to vary the length of said predetermined time interval for varying the effect of the rate of acceleration in rendering said mechanism non-operative.

2. A system for controlling the speed of a power plant driving a propeller, including an internal combustion engine having an intake manifold, a propeller the blades of which are adjustable as to pitch, a mechanism to adjust blade pitch to change the speed of said power plant from one speed to a different predetermined speed, means responsive to the actual speed of said power plant and in accordance with the rate of acceleration thereof for rendering said mechanism non-operative at a predetermined time interval before said power plant attains said predetermined speed level, and means responsive to manifold pressure to vary the length of said predetermined time interval for varying the eifect of the rate of acceleration in rendering said mechanism nonoperative.

3. A system for controlling the speed of a power plant driving a propeller, the blades of which are adjustable a to pitch, a mechanism to adjust blade pitch to change the speed of said power plant from one speed to a different predetermined speed, means responsive to the actual speed of said power plant and in accordance with the rate of acceleration thereof for rendering said mechanism non-operative at a predetermined time interval before said power plant attains said predetermined speed level, and means responsive to manifold pressure and airspeed to vary the length of said predetermined time interval, for varying the effect of the rate of acceleration in rendering said mechanism non-operative.

11 4. A system for controlling the speedof a power plant driving a propeller, the blades of which are adjustable as to pitch, a mechanism to "adjust blade pitch, said system having aoertain order of lag in responding to calls for speed change, means for producing a voltage characteristic of the actual speed of said power plant, means responsive to said voltage for initiating the operation of said mechanism to correct the speed of said power plant, mean responsive to said last mentioned means for feeding another voltage back and combining it with said first mentoined voltage during correction periods, thereby to damp operation of said voltage responsive means in consonance with the lag in the response of said power plant, a device sensitive to and operative in response to a power plant operating characteristic independent of power plant rotational speed, and means for varying said other voltage in response to operation of said device.

5. A system for controlling the speed of apower plant driving a propeller, the blades of which are adjustable as to pitch, a mechanism to adjust blade pitch, said system having a certainorder of lag in responding to calls for speed change, means for producing a voltage characteristic of the actual speed of said power plant, means responsive to said Voltage for initiating the operation of said mechanism to correct the speed of said power plant, means responsive to said last mentioned means for feeding another voltage back and combining it with first mentioned voltage during correction periods, thereby to damp operation of said responsive mean in consonance with the lag in the response of said power plant, and means for varying said other voltage in response to airspeed.

6. A system for controlling the speed of an internal combustion engine driving a, propeller, the blades of which are adjustable as to pitch and are actuated by a mechanism, said engine having lag in speed change response, said system including means for producing a voltage characteristic of the actual speed of said internal combustion engine, means for producing a second feedback voltage, a time-delay circuit through which said feedback voltage may be combined with said first mentioned voltage during correction periods to damp circuit operation in consonance with the lag in the response of said internal combustion engine, means for altering said combined voltage in response to a reduction in manifold pressure below a normal pressure range, and means responsive to the combined voltage for controlling the operation of said I mechanism.

7. A system for controlling the speed of an aircraft power plant driving a propeller, the blades of which are adjustable as to pitch and are actuated .by a mechanism, said power plant having lag in speed change response, said system including means for producing a voltage characteristic of the actual speed of .said power plant, means responsive to said voltage for initiating the operation of said mechanism to correct the speed of said power plant, means for feeding a voltage back and combining it with said first mentioned voltage during correction periods, thereby to damp the operation of said voltage responsive means in consonance with the lag .in the response of said power plant, and means for modifying said combined voltage in response to aircraft airspeed conditions.

8. A system for controlling the speed of an internal combustion engine driving a propeller, the blades of which are adjustable as to pitch and are actuated by a mechanism, said engine having lag in speed change response, said system including means for producing :a voltage character'- istic of the actual speed of said englnameans 101' producing a second feedback voltage, a time delay circuit through which said feedback voltage may be combined with said first mentioned vottage during correction periods to damp circuit opera-'- tion in consonance with the lag in the response of said engine, means for altering said combined voltage in response to a change in power of said engine, and means responsive to the combined voltage for controlling the operation of said mechanism.

PAUL J. QUINN.

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